Blacklist

In the computer world, a blacklist is a way to filter Internet traffic so that all traffic is allowed except that from servers on the blacklist. This tool is usually used to counter spam.

To use a blacklist to filter out spam, an IT administrator must set a blacklist of servers that are known senders of spam. All incoming email traffic is then checked against that list to remove spam. This means that even legitimate email from service providers who allow spammers to use their services will be blocked.

A related concept is a whitelist, which only allows incoming traffic from users on the whitelist, rejecting all other traffic. This tool is usually used for firewalls.